Oxytocin is a peptide hormone. It stimulates contraction of the uterine muscles. For this reason, it is believed to be involved in the etiology of pre-term labour and menstrual pain. It is further believed that oxytocin antagonists would be useful in the control of these conditions. Oxytocin antagonist peptides of adequate potency and selectivity of therapeutic use are known. They are often intended for administration in aqueous solution. The manufacture of ready-for-use doses of such antagonists requires that the solutions be stable for extended periods, which is not always true. In such cases, the medicament must be prepared immediately prior to use from, for instance, the freeze-dried peptide or its pharmaceutically acceptable salt. This sort of manipulation is inconvenient and entails the risk of contamination.